In period, Christianity was very much focussed on the veneration of relics and pilgrimage to holy places. As the tomb of Christ, the Holy Sepulcher was seen as the holiest single location in Christianity. When access to the Church of the Holy Sepulchre was seen as threatened by the Muslims, Christians in Europe launched the Crusades to reclaim them and (among other things) guarantee access.
The Holy Sepulchre was so important to the Crusaders that when the Kingdom of Jerusalem was founded in 1099 after the First Crusade, its first ruler, Godfrey of Bouillon, took not the title of King, but instead was styled Advocatus Sancti Sepulchri -- "Defender of the Holy Sepulchre."
Like many Christian pilgrimage sites in the Holy Land, the location of the Sepulchre was identified by Helena of Constantinople, mother of Constantine I, who caused a church to be built over the tomb.
Subsequently either all or the vast majority of the edifice was torn down, and it was not until the Christian re-occupation that a new church was built over the suppsoed site, which also extends to the site of the Crucifiction (Golgotha), and to the place where the True Cross was found by Helena.